A head or skull clamp of this type, in which the one holder arm has a two-arm mandrel holder, which is rotatable in an axial bore of the holder arm by means of a special bearing device and can be fixed in different rotated positions, has already become known from DE 694 11 621 T2 (=EP 0 623 318 B1). The two-arm mandrel holder is mounted in a fork-shaped holding device to pivot about a pivot axis running transversely to the bearing shaft.
A single mandrel opposing the two-arm mandrel holder is mounted in a threaded spindle, which is inserted in the opposite holder arm coaxially to the bearing shaft of the two-arm mandrel holder. The threaded spindle can be adjusted axially by rotating by means of a rotary knob designed as a knurled disk, such that the distance of this single mandrel from the two mandrels of the two-arm mandrel holder is variable and can be adjusted to the patient's head to be fixed in each case. The single mandrel can be mounted in a spring-mounted manner in the threaded spindle in the axial direction.
In this prior-art head clamp, the connecting bar connecting the two holder arms, lying in a common plane, has a two-part design and can be lengthened or shortened in its direction of extension, such that a coarse change in distance is possible between the single mandrel and the two mandrels of the two-arm mandrel holder.
A bracket provided with a toothed ring, by means of which the entire head clamp can be fastened at a frame in various pivoting positions, is provided at one part of the connecting bar. In this case, the axis of the toothed ring, about which the entire head clamp can be pivoted and fixed in the one or other pivoting direction, runs at right angles to the common bearing shaft of the threaded spindle and of the bearing of the two-arm mandrel holder.
However, it has arisen in practice that two-arm mandrel holders on the top side opposite the single mandrel are not optimal for all applications, and that the two-part design of the connecting bar, by means of which the coarse distance adjustment can be made, brings with it an awkward handling as well, which requires improvement.
Other head clamps have also become known from U.S. Pat. Nos. 3,835,861, 4,169,478 and 5,254,079. In these as well, only one two-arm mandrel holder is provided on the side opposite the single mandrel, respectively. The connecting bar has a two-part design for the coarse distance adjustment. In this case, a part of the connecting bar is provided with sawtooth-like locking teeth, with which a locking member provided with locking teeth, meshes in a spring-mounted, detachably locking manner.
In all these head clamps, the mandrel holder of the single mandrel is mounted in the cylindrical cavity of a threaded spindle in the form of a piston that is displaceable against spring pressure and is provided on the front side with a socket bore for receiving the single mandrel. On the front side facing away from the single mandrel, a stop screw is screwed into the holder shank, whose head with a supporting disk lies against the front surface of a knurled gripping part. Thus, an axial quick adjustment with abolishing of the threaded contact is not possible.
Head clamps, in which the hollow spindles, into which the mandrel holders are axially displaceably mounted against spring pressure by means of threaded contacts, have external locking teeth, with which detachable locking elements mesh in a fixed manner, have become known from DE 197 18 535 C2 and DE 20 2004 006 726 U1. By loosening the locking elements, the hollow spindles are freely displaceable in their bearing bores.